One well-known type of phase lock loop combines a loop input signal with a variable frequency output signal of a voltage controlled oscillator. The loop input signal and voltage controlled oscillator output signal are applied to a mixer which derives a difference frequency that is applied to a low pass filter, frequently referred to as a loop filter. The loop filter drives a control input terminal of the voltage control oscillator with a signal having an AC component with a frequency equal to the frequency difference between the frequencies of the loop input signal and the voltage controlled oscillator output signal. When the loop input signal and oscillator signal frequencies are the same, the loop filter supplies the oscillator control input terminal with a DC component representing the phase difference, over a limited range, between the input signal and the voltage controlled oscillator output signal.
The mixer output signal is a phase error signal which is a sinusoidal function of the actual phase error between the loop input signal and the output signal of the voltage controlled oscillator. In consequence, when the typical prior art phase lock loop is acquiring a signal, the loop displays a non-monotonic characteristic, frequently causing the loop to slip many cycles before achieving phase lock between the loop input signal and the voltage controlled oscillator output signal.
The non-monotonic characteristic of the typical prior art phase lock loop occurs because of the sinusoidal nature of the phase error output signal of the mixer when the loop input signal differs in frequency from the voltage controlled oscillator output signal. The sinusoidal phase error signal results in an oscillatory acquisition behavior of the typical prior art phase lock loop. If the loop is designed as a first order loop, as determined by the characteristics of the loop filter, there is a maximum frequency offset for which the loop can never achieve lock because the phase error signal can only be generated over a limited range. While a second order loop is capable, in principle, of achieving phase lock between the loop input signal and the voltage controlled oscillator output, implementation considerations limit the practical pull-in range to something on the order of the loop bandwidth or slightly greater for loop input signals having high signal-to-noise ratios.
While the acquisition range of a phase lock loop can be increased by increasing the loop filter bandwidth, this has the deleterious effect of increasing the signal-to-noise ratio requirements for a detectable loop input signal. Hence, one of the very purposes of a phase lock loop, i.e., to provide a very narrow bandwidth detector for signals having low signal-to-noise ratios, is obviated by increasing the loop bandwidth.
It is, accordingly, an object of the present invention to provide a new and improved phase lock loop capable of acquiring a signal over a relatively wide bandwidth, without reducing the loop detection characteristics.
Another object of the invention is to provide a new and improved phase lock loop having a monotonic output function relating the frequency difference between loop input and output signals.
An additional object of the invention is to provide a phase lock loop having a non-oscillatory acquisition behavior in response to input signals having a relatively wide spread of frequencies
Still another object of the present invention is to provide a new and improved phase lock loop having a relatively wide frequency pull-in range and a relatively wide lock-in range.